A customer left me with just the crankset from a 12-speed Altegra FC-R8100.
In the article below, I'll refer to the direction from the crank axle to the pedal hole on the main arm as "down," and the side where the 4-arm phase is spread wide (upper right in the image above) as "up."

Since they're racing cyclocross,
they want me to install an 11-speed 46-36T on this.

Both the FC-R8100 and FC-R8000 come in 46-36T configurations,
but with the R8100, I couldn't find a single 46T chainring
available for individual sale from what I could tell.

So what happens if the 46T chainring on the R8100 wears out?
The real answer is coming later.


I ordered in an R8000 46T.
An R8100 46T would definitely fit the crank,
but this bike's components are 11-speed.
However, with an R8000 46T,
there's no guarantee it will fit the R8100 crank.

Also, starting with the 7900 Dura-Ace series,
the outer chainring design became continuous with the crank arm,
but with the 46T, it has the flat plate shape of older outer gears.


This peculiar shape at the lower right, using the orientation I established earlier,
has been common since the 4-arm design.


↑R8100 crank lower right side


The width between the chainring and crank arm doesn't match,
but the shape of the inner contact surface fits perfectly.


However, only the lower left doesn't fit at all.
A protrusion on the chainring is in the way.


Looking at the protrusion from the back, there's a cutout.
If this situation were reversed—crank being R8000 and outer chainring being R8100—
I'd be thinking "They added a protrusion to prevent older chainrings from being installed!"
But actually, it's the opposite.

However, this shape does seem to be an effort to prevent R8000 outer chainrings
from being installed on R8100 cranks.
In other words: "They changed the crank arm shape so the protrusion blocks installation
to prevent older chainrings from being fitted!"

What caught my attention is that the protrusion that won't fit the R8100
seems to match perfectly with the cutout.
If that's the case, then the R8100 chainring is basically
the R8000 chainring with this cutout section removed.
No way they'd make such a crude specification change just by doing that...


But even thinking that, I ground away just the cutout section,


And it fit perfectly.


It's tricky to thread it through like a puzzle,
but aside from the lower left, it mounted on the crank without any modification.
So the specification change was basically: "If we just change the lower left shape,
most people will give up trying to install it."

This is only possible because the 46T is a flat plate—unlike outer gears of other tooth counts
that are integrated as part of the 4-arm shape.


The outers with the crank-continuous design have the fixing bolt nut
embedded on the gear side, but
both the R8100 and R8000 46T come with a dedicated nut
plus fixing bolt for the crank-continuous design.
Since the crank is R8100,
I ordered the R8100 version.


The threaded section is anodized blue.
This color-coding is probably to prevent mistakes during manufacturing and packaging.


Actually, when you tighten the fixing bolt as shown above,
the blue section isn't visible at all.


↑Upper right and lower right


↑Upper left and lower left
Being genuine parts, the crank-continuous design
fits perfectly.


This job involves the unorthodox practice of installing
an 11-speed chainring on a 12-speed crank.
However, while this crank is an FC-R8100, I thought there might be cases where
the similar FC-R8 (the free inspection program
—they stubbornly refuse to call it a recall—) replacement crank
needs the original FC-R8000 46T installed.
So I wrote this article.


Earlier I said "There's no R8100 46T single-part sale,"
but upon further research, this is because
the R9200 Dura-Ace also got a 46-36T option,
and they share the outer chainring.
So when the R8100 46T wears out,
you'd buy an R9200 46T instead.
On this 46T, where other tooth counts say "DURA-ACE,"
it says "SHIMANO" instead.
In contrast, the R8000 46T in this case
is Altegra-specific, so it's marked "ULTEGRA."

As of this writing, the tax-included list price
for the R8000 46T is ¥13,749.
Meanwhile, the R9200 (R8100) 46T is ¥22,968 tax-included.
The complete FC-R8100 46-36T crankset is ¥40,950,
so the replacement outer chainring price
exceeds 50% of the crankset cost.

Outer chainrings other than the 46T have the crank-continuous shape
integrated with the 4-arm, so prices are higher than flat plate chainrings—that's unavoidable.
When I checked the tax-included list prices for FC-R9200 versions,
50T: ¥27,115
52T: ¥27,753
54T: ¥36,175
So the flat-plate 46T at ¥22,968 seems high
even accounting for low production volume driving up costs,
and it feels even more expensive when used for Altegra repairs
rather than Dura-Ace.

Swapping between 50T, 52T, and 54T uses the same four fixing bolts,
and even replacing them new costs ¥1,444 tax-included.
But the crank-continuous design nut (4 pieces) plus bolt (4 pieces) set
for the 46T has a tax-included list price of:
R9200 version: ¥3,069
R8100 version: ¥3,069 (separate part from R9200)
R8000 version: ¥4,638 (why is this the most expensive?)
So if you're buying the continuous-design nuts and bolts together
when upgrading from other tooth counts to 46T,
the price isn't much different from swapping 50T or 52T with reused bolts.

Finally, the tax-included list price for FC-R8100's crank-integrated
outer chainring is: ¥14,738 for both 50T and 52T.
The flat-plate 46T at ¥22,968—though unmarked—must be
a brand premium for being Dura-Ace grade.
Some time ago I wrote about how XOTR hubs moved from Japanese to Malaysian manufacturing
but the price gap with XT hubs remained, suggesting it's just expensive "because it's XOTR,"
and this might be the same situation.

For the complete FC-R8100 crankset, both 50-34T and 52-36T
have a tax-included list price of ¥39,449,
while the 46-36T is ¥40,950—not as large a price difference
as the single chainring.
Man, this makes no sense.